1. Field of the Invention
The present invention relates to interchangeable lenses with different amounts of protrusion from a mount reference surface toward an image plane used for a camera system including a plurality of cameras having the same mount structure, and the camera system.
2. Description of the Related Art
With the advance of digitization of cameras in recent years, there is a proposal on a single-lens reflex camera system whose lenses are interchangeable made up of single-lens reflex digital cameras which inherit the sameness in a mount shape to allow conventional interchangeable lenses to be used on one hand, and which replace a 35 mm silver-haloid film by an electronic image-pickup element such as a CCD sensor and CMOS sensor However, an electronic image-pickup element such as CCD sensor or CMOS sensor having the same size as the 35 m silver-haloid film is very expensive, and therefore the use of an image-pickup element (e.g., APS-C size image-pickup element) is becoming the mainstream instead of the 35 mm film.
Since such a single-reflex digital camera using an image-pickup element smaller than a conventional 35 mm film has a smaller image circle than the 35 mm film, it is possible to reduce the size of a quick return mirror in the camera body and consequently reduce the size of a camera body itself. Furthermore, when the quick return mirror becomes smaller than the conventional one, it is also possible to shorten the back focus of its interchangeable lens than the conventional interchangeable lens and its image circle is also small, which produces the merit of downsizing the interchangeable lens.
However, once the aforementioned short back-focus interchangeable lens is mounted on a 35 mm film camera body or a digital camera supporting an image circle larger than the interchangeable lens, the back end of the interchangeable lens may interfere with the quick return mirror. Furthermore, allowing an interchangeable lens dedicated to a digital camera having a short back focus (with a small image circle) to be mounted on the camera body of a conventional system causes problems such as a luminous flux failing to reach the periphery of the image-pickup surface.
Therefore, there is a necessity for means for preventing the camera body and interchangeable lens from being mounted on such a combination.
Japanese Patent Publication No. 3217273 discloses a camera system, camera body and interchangeable lens which have a common mount but imposes restrictions on the mounting of interchangeable lenses having a back focus of a certain size or smaller. This camera system, etc., provides a flange portion inside the mount of the conventional camera body and restricts the mounting of the interchangeable lens by causing a fixed protrusion provided on the interchangeable lens with a short back focus to contact this flange portion.
Furthermore, Japanese Patent Publication No. 3217273 also discloses the fact that it is possible to reduce the size of a quick return mirror of a camera with a short back focus compared to a quick return mirror of a camera with a long back focus.
However, as disclosed in Japanese Patent Publication No. 3217273, it is not possible to bring the rotation area (rotation track) of the quick return mirror sufficiently close to the image-pickup surface by only reducing the size of the quick return mirror. For this reason, the protruding amount from a mount reference surface of a dedicated interchangeable lens mounted on this camera to an image-pickup surface is limited and it is not possible to shorten the back focus sufficiently.